stray voltage across 120vac LED indicator bulb

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kennybobby
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stray voltage across 120vac LED indicator bulb

This post is only related to LED bulbs in that this industrial control cabinet circuit uses miniature bayonet green LED bulbs marked “120V” no other markings that i could find. Something is causing a stray voltage and the LEDs are very faintly lit anytime the mains are connected. Voltage readings with and without the bulbs are marked on the wiring diagram.

i couldn’t get a voltage drop reading with the DMM diode check function; they turn ON faintly with the probes in either direction, there are no polarity markings. i’ll have to take a picture to post up, i figured i could find it online but no luck. i think it’s like a BA9 miniature base.

There shouldn’t be any current flow by the diagram, but i saw them glowing, L1 was brighter than the others.

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Lightbringer
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Probably leakage in/around the switches.

All switches in the positions as shown? Ie, S1/S2/S3 all open?

09 F9 11 02 9D 74 E3 5B D8 41 56 C5 63 56 88 C0

kennybobby
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yep all switches open

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Lightbringer
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Wellp, if mechanical switches, That Could Be Bad.

If electronic switches (triac, etc., like in a dimmer-switch, even if used on/off), there’ll always be some leakage.

I got a bunch of X10 switches and dimmers, and plug a CFL bulb into a socket controlled by one of those, even if you keep in mind to never ever ever dim it, just off/on, and it’ll blink faintly like flea-sized lightning when the switch is nominally off. It’s actually on, but at a very very high resistance.

Anyhoo, once S1 is off, that should be it, zero current anywhere past the switch.

Now, it’s possible — unlikely, but possible — that if any wires are long enough, they could be inducing a stray voltage like an antenna, especially if run alongside any live-power lines, or near/around the transformer (like a 1-turn secondary).

09 F9 11 02 9D 74 E3 5B D8 41 56 C5 63 56 88 C0

kennybobby
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i scanned inside the cabinet with a Magnetic Field probe (small 1” diameter antenna loop) while S1 was open and i could see no signal on the o’scope. This makes sense since there is no current flowing when the circuit is OFF.

Then i used an Electric Field probe (short monopole antenna) and was picking up stray 60 Hz AC in the vicinity of all the wiring going to the switches and lights. It was the strongest when near the Primary wires running to the transformer, which makes sense since that has the highest voltage swing of 480 vac between each other and about 240 vac each to earth ground chassis.

i put a 100k resistor across the light terminals on L1 and the DMM voltage measurement dropped to about 0.121 vac and the LED no longer was lit. This would solve the dimly lit when OFF issue, but there should really be an ON/OFF disconnect for the 3-phase mains to cut off power to the primary and a fuse in the wires to the primary.

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